1. Field of the Invention
This invention relates to an expansion shell assembly and method for anchoring a roof bolt in a bore hole containing resin material and more particularly to an expansion shell assembly adapted for use with resin material to reduce slippage of the assembly in the bore hole and thereby maintain the tension on the bolt.
2. Description of the Prior Art
In underground operations, such as mining or excavating, the unsupported rock formation is supported by bolt members that are inserted in a bore hole drilled in the rock formation and are secured thereto by engagement of an expansion shell on the end of the bolt with the rock formation. A roof plate is retained on the end of the bolt that extends from the bore hole and abuts the roof. Rotating the bolt having an expansion shell positioned on an opposite end thereof expands the shell to engage the rock wall of the bore hole. The bolt is tensioned with the resultant effect of compressing the rock strata and thereby reinforcing the strata to resist forces that apply shear stresses to the rock formation. However, in soft rock strata over a period of time slippage occurs of the expanded shell in the bore hole. Consequently the tension on the bolt reduces, and the roof support provided by the bolt is decreased.
Supporting a rock formation by adhesively bonding an elongated bolt in a bore hole is illustrated and described in U.S. Pat. Nos. 3,324,662 and 3,394,527 and includes adhesively bonding the bolt in position by a thermosetting resin composition having thixotropic properties. The resin composition includes principally two components, a polyester resin and a catalyst, that are separately retained in a cartridge. One or more cartridges are suitably positioned in the bore hole and are fractured by upward movement of the rod which is rotated to intimately admix the components to form an essentially homogeneous curable resin mixture. The resin mixture polymerizes at ambient temperature and penetrates into the surrounding rock formation to adhesively unite the rock strata and to firmly hold the bolt in position in the bore hole. The resin mixture fills the annulus between the bore hole and the rod along a substantial length of the rod.
The principal disadvantage of the mechanical roof bolt apparatus is that contact between the roof bolt and the rock formation is confined to engagement of a relatively narrow portion of the fingers of the shell with the bore hole wall. Thus when the bolt is rotated to tension the bolt and compress the rock strata, particularly in soft strata, a loss in tension results due to slippage of the expanded shell in the bore hole. U.S. Pat. No. 3,702,060 discloses an expansion shell assembly that includes a resin container which is fixed to the end of the expansion shell and is ruptured upon actuation of the expansion shell to mix the components. The mixed components flow downwardly in the shell and are confined to the area immediately around the shell where the material reinforces the local rock strata and bonds the shell to the strata. The bolt is rotated until it reaches 80% of the final tension before the cured resin prevents further rotation of the bolt. When the resin is fully cured, a nut on the end of the bolt opposite the roof plate is rotated to bring the roof plate to its fully seated position against the mine roof to fully tension the bolt.
U.S. Pat. No. 3,188,815 discloses an anchor bolt assembly in which the tip of the bolt is cemented to the bottom of the drill hole by means of a plastic or cement mixture. The cement is mixed by driving in and rotating the bolt. An expanding wedging element is spaced from the tip of the bolt and is sealed in the bore hole from the mixed cement by a sealing disc which prevents the cement from contacting the wedge. After mixing the cement, further rotation of the bolt expands the wedge to tension the bolt. U.S. Pat. Nos. 3,222,873 and 3,695,045 disclose bonding anchor bolts, rods and the like to the wall of a bore hole by a resin mixture in which an expansion shell is utilized to hold the bolt in place until the resin cures without tensioning the bolt.
U.S. Pat. Nos. 3,877,235 and 4,051,683 disclose mine roof support devices that include elongated bolt members adhesively secured in the bore hole be a suitable resin material. In the former patent, mixed and hardened adhesive material non-rotatably secures a hollow pipelike anchor member in the bore hole. A bolt member is rotatably advanced through the anchor until a bearing plate is moved into contact with the mine roof and the bolt is tensioned. In the latter patent, a rod member is anchored in the bore hole by a resin material and is connected by a coupling to a bolt that includes a bearing plate on one end of the bolt opposite the mine roof. When the resin cures and the rod is bonded to the rock formation, further rotation of the bolt through the coupling advances the bolt upwardly in the bore hole to tension the bolt having the roof plate abutting the mine roof.
It has been suggested by the prior art devices to support a rock formation, such as a mine roof, by mechanically supporting bolts within the bore holes and/or adhesively bonding the bolts to the rock formation. The mechanically anchored roof bolt is susceptible to a loss of tension in the bolt by slippage of the expansion shell in the bore hole. The adhesively bonded bolt is not capable of being tensioned. Therefore, there is need for an expansion shell assembly that is operable for use with adhesive material to prevent slippage of the expansion shell in the bore hole and thereby maintain tension on the bolt.